Many diseases of procaroytic origin, i.e. caused by pathogenic bacteria, are known. Such diseases are in general more easily treated than those of eucaryotic origin because of the marked differences between the invading procaryotes and the eucaryotic cells of the host. Thus, because of the differences between bacterial cells and those of the host, many antibiotics are known to specifically inhibit the invading bacteria without significant adverse effects on the host. In contrast, it has generally been more difficult to treat diseases of nonbacterial origin, such as malaria, sleeping sickness and Chagas disease.
The property of certain peptides to induce lysis of procaryotic microorganisms such as bacteria are known. For example, U.S. Pat. Nos. 4,355,104 and 4,520,016 to Hultmark et al describe the bacteriolytic properties of some cecropins against Gram-negative bacteria. Quite interestingly, the cecropins described in the Hultmark et al patents were not universally effective against all Gram-negative bacteria. For example, the cecropins described therein lysed Serratia marcescens D61108, but not Serratia marcescens D611. Moreover, cecropins have heretofore been reported to have no lytic activity towards eucaryotic cells such as insect cells, liver cells and sheep erythrocytes, as reported in the Hultmark patents; International Patent Publication WO/8604356; Andreu et al, Biochemistry, vol. 24, pp. 1683-88 (1985); Boman et al, Developmental and Comparative Immunology, vol. 9, pp. 551-558 (1985); and Steiner et al, Nature, vol. 292, pp. 246-248 (1981).
Other lytic peptides heretofore known include, for example, the sarcotoxins and lepidopterans. Such peptides generally occur naturally in the immune system of Sarcophaga peregrina and the silkworm, lepidopteran, respectively, as reported in Nakajima et al, The Journal of Biological Chemistry, vol. 262, pp. 1665-1669 (1987) and Nakai et al, Chem. Abst. 106:214351w (1987).
The mechanism of action of the lytic peptides in the immune systems in which they occur is not entirely clear. There must, of course, be some aspect of the mechanism which regulates the specificity of the lytic peptides for invading pathogens among the cells of the host organism which must generally be preserved from lysis. For example, human complement fixation involves antibodies which are generally specific for certain antigens expressed by the invading pathogen. The activated components of complement attack the membrane of the invading cell to which they are bound by the antigen-antibody reaction to produce circular lesions which are probably formed as a result of insertion of the C9 protein into the membrane. The more primitive mechanisms involved in insect immunology are less specific, but the peptides involved apparently do not significantly lyse the host cells.
There are many differences between the membranes of different types of cells which can affect their susceptibility to lysis by the various lytic peptides. As suggested above, for example, some proteins are capable of lysing only cells expressing an appropriate antigen for the antibody associated with such protein. Thus, it is not surprising that the less specific lytic peptides such as cecropins are more capable of lysing procaryotes than the eucaryotic cells of the insect.
Gram-positive procaryotes generally have a thicker cell wall than Gram-negative ones. Also, Gram-positive cell membranes have a cytoplasmic membrane and a cell wall containing mostly peptidoglycans and teichoic acids, whereas Gram-negative cell membranes have an inner cell wall consisting entirely of peptidoglycan and associated proteins surrounded by an outer cell wall comprised of lipid, lipopolysaccharide and protein. In contrast, eucaryotic cells generally have a plasma membrane comprising a lipid bilayer with proteins and carbohydrates interspersed therein, and also have organelles with their own membrane systems, but generally do not have an outer cell wall. It is readily appreciated that the considerable variation of membrane structures among bacteria (procaryotes) accounts for considerable variation in their susceptibility to lysis by the various insect immune proteins.
The variation of membrane structures among eucaryotes is also considerable, but these membranes generally comprise phospolipid molecules in a bilayer arrangement with a thickness of about 50 .ANG.. The hydrophilic portion of the phospholipid is generally oriented to the exterior and interior surfaces of the membrane, while the hydrophobic portions are generally found in the interior region of the membrane between the hydrophilic surfaces. As reported in Nakajima et al, the presence of cholesterol and the assymetric distribution of phospholipids in the cytoplasmic membrane of eucaryotic cells may explain the selective toxicity of sarcotoxin to bacteria. Since cholesterol causes condensation of the phospholipid bilayers, it can hinder the penetration of lytic peptides into the cytoplasmic membrane of eucaryotic cells. Similarly, the predominance of neutral phospholipids in the outer monolayer of eucaryotic membranes would result in less affinity to positively charged lytic peptides such as cecropin and sarcotoxin than acidic phospholipids generally located on the cytoplasmic side of the membrane.
A number of the antibacterial polypeptides have been found to be useful when the genes encoding therefor are incorporated into various plant species. Particularly, when introduced into the plant genome by means of an Agrobacterium plasmid vector, the antibacterial polypeptide-encoding genes produce plant species much more resistant to certain bacterially induced disease conditions and plant pathogens. Such antibacterial polypeptides and the transformation of plants with genes encoding therefor are described in aforementioned U.S. patent application Ser. No. 889,225.
Polynucleotide molecules expressible in a given host and having the sequence araB promoter operably linked to a gene which is heterologous to such host are also known. The heterologous gene codes for a biologically active polypeptide. A genetic construct of a first genetic sequence coding for cecropin operably linked to a second genetic sequence coding for a polypeptide which is capable of suppressing the biological effect of the resulting fusion protein towards an otherwise cecropin-sensitive bacterium is also described in International Publication W086/04356, Jul. 31, 1986.
The Hultmark et al patents mentioned above also mention that there are no known antibodies to cecropin, indicating a wide acceptability for human and veterinary applications, including one apparently useful application for surface infections because of the high activity against pseudomonas. Similarly, EPO publication 182,278 (1986) mentions that sarcotoxins may be expected to be effective in pharmaceutical preparations and as foodstuff additives, and that antibacterial activity of sarcotoxin can be recognized in the presence of serum. Shiba, Chem. Abstr. 104: 230430K (1985) also mentions preparation of an injection containing 500 mg lepidopteran, 250 mg glucose and injection water to 5 ml.
Several analogs of naturally-occurring cecropins, sarcotoxins and lepidopterans have been reported. For example, it is reported in Andreu et al, Proc. Natl. Acad. Sci. U.S.A., vol. 80, pp. 6475-6479 (1983) that changes in either end of the amino acid sequence of cecropin generally result in losses in bactericidal activity in varying degrees against different bacteria. It is reported in Andreu et al (1985) mentioned above that Trp.sup.2 is clearly important for bactericidal activity of cecropin, and that other changes in the 4, 6 or 8 position have different effects on different bacteria. From the data given in Table II at page 1687 of Andreu et al (1985), it appears that almost any change from natural cecropin generally adversely affects its bactericidal activity. Cecropin is defined in International Publication W086/04356 to include bactericidally active polypeptides from any insect species and analogs, homologs, mutants, isomers and derivatives thereof having bactericidal activity from 1% of the naturally-occurring polypeptides up to 100 times or higher activity of the naturally-occurring cecropin. Other references generally discuss the effects of the .alpha.-helix conformation and the amphiphilic nature of cecropin and other lytic peptides.
It is known that lysozyme and attacins also occur in insect hemolymph. For example, it is reported in Okada et al, Biochem. J., vol. 229, pp. 453-458 (1985) that lysozyme participates with sarcotoxin against bacteria, but that the bactericidal actions are diverse. Steiner et al mentioned above suggests that lysozyme plays no role in the antibacterial activity of insect hemolymph other than to remove debris following lysis of bacteria by cecropin. Merrifield et al, Biochemistry, vol. 21, pp. 5020-5031 (1982) and Andreu et al (1983) mentioned above state that cecropin purified from insect hemolymph may be contaminated with lysozyme, but demonstrate that the synthetically prepared cecropin is as bactericidally active as purified cecropin from insect hemolymph.